1. Field of the Invention
The subject invention relates generally to borehole centralizers and is specifically directed to a self-contained, self-energizing centralizer system.
2. Discussion of the Prior Art
Centralizers are devices used to provide space between an interior member and the walls of a bore hole, thereby maintaining a concentric assembly with a uniform annulus between the interior member and the bore hole wall. The centralizer field includes many different inventions which have attempted to improve the functions of the centralizer. Its two primary functions are to: 1) provide space between the anchorage and the walls of the bore hole, and 2) to provide a locking mechanism for the several centralizer elements so that they do not fail once installed.
In certain oilfield applications, in particular in dual barrier deepwater production risers, it is necessary to rigidly centralize and locate an inner casing string within an outer riser. There may be up to 50 centralizers at various heights up the riser all within the annular space. The centralizers must be kept un-activated while the casing is installed in the riser, to prevent damage and hang-ups on restrictions to the riser bore in the wellhead.
The usual way of achieving this is some sort of hydraulically activated device, where each centralizer is manifolded to a hydraulic line which must exit the annulus at the wellhead so that it can be controlled. This has the disadvantages of a large number of potential leaks, expense and complication of contracting and assembling the system, and potential for causing a leak path later in field life.
Known relevant prior art has focused on improving the locking mechanism. Prior art designs have had some success in overcoming some of the problems inherent with centralizer systems, including inflexibility in meeting multiple applications and relatively complex locking mechanisms. However, the prior art virtually neglects the problem of complex locking systems and the requirement that the lock activator exit the annulus at the wellhead in order to be controlled.
As is well known in the art, the collars of such centralizers normally comprise hingedly connected, arcuate sections which may be opened up for assembly about the casing and then latched to one another in close fitting relation thereabout. The bow springs comprise relatively thin, flexible strips of metal which are bowed outwardly intermediate their ends so as to tightly engage the bore of a well. More particularly, the bow springs are equally spaced apart so as to hold the casing in a generally centered position within the well bore to insure that the column of cement forced into the annulus between the casing and well bore is of substantially uniform thickness. The collars fit closely about the casing, but are free to slide therealong, so that, when the central portions of the bow springs are inwardly compressed by engagement with the well bore, their ends move the collars away from one another.
Many patents including U.S. Pat. No. 6,367,556, U.S. Pat. No. 4,909,322, U.S. Pat. No. 4,651,823, U.S. Pat. No. 4,520,869, U.S. Pat. No. 4,269,269, U.S. Pat. No. 4,077,470 and U.S. Pat. No. 4,042,022 describe a centralizer device which comprise a plurality of spring bows, a method of attaching the bow springs while still allowing flexure, and a method of fixing the centralizer device to a drill-pipe or casing. The bow springs allow the centralizer to pass down into the bore, and pass small irregularities and restrictions in the bore, while still maintaining some centralizing load.
Other patents including U.S. Pat. No. 4,247,225 and U.S. Pat. No. 4,866,903 disclose an alignment devices comprising a fixed portion for the purpose of centralizing a component in a bore. These devices offer a rigid centralization inside a true bore, but cannot pass through small irregularities and restrictions in the bore
U.K. Pat. No. GB 2,381,280 describes a centralizer with a plurality of moveable ribs which can be activated to centralize the casing or pipe inside a bore. This device requires manual activation, and moves the ribs out to a position where they are locked in place and functions like a fixed diameter centralizer. It is conceivable that this invention could be converted to use in an annular space by configuring hydraulic cylinders to activate the centralizer mechanism. This has the disadvantage of requiring hydraulic lines between each centralizer device, and a means for exiting the hydraulic control line from the enclosed annular space.
It would be desirable to provide a locking system that can be energized without invading the bore and without requiring extensive energization systems.